In a multiplicity of industrial and municipal applications, such as wastewater purification and seawater desalination, membrane-supported filtration methods, in particular crossflow filtration, have been used for decades. Liquid that is to be purified—hereinafter called feed—flows over two-dimensional porous membranes tangentially to the membrane surface. The pore size of the membranes, depending on the application, is in the range from about 10 nanometers to some micrometers. The volume of the feed that flows through, customarily termed flow, is separated from a permeate space by the membrane. Between flow and permeate space a differential pressure of about 0.1 bar to 100 bar is applied which causes a mass transport from the flow to the permeate space, wherein permeate (or filtrate) passes into the permeate space. For the membrane bioreactors (MBR) used in wastewater treatment, preferably a differential pressure in the range from 0.02 to 0.4 bar is employed.
The membrane is usually constructed as a two-layer composite of a support nonwoven and a porous membrane layer. Preferably, the porous membrane layer comprises polyether sulfone, polysulfone, polyacrylonitrile, polyvinylidene fluoride, polyamide, polyetherimide, cellulose acetate, regenerated cellulose, polyolefin or fluoropolymer. The porous membrane layer is generated, for example, by coating a nonwoven or woven fabric with polymer solution and precipitating out the polymer in a subsequent phase inversion step. Alternatively thereto, a polymer film is stretched in a suitable manner, wherein pores are formed in the polymer film. The stretched polymer film is then laminated onto a support nonwoven for mechanical stabilization. Filtration membranes produced by these methods are commercially obtainable, e.g. under the name NADIR® membranes (MICRODYN-NADIR GmbH, Wiesbaden) or CELGARD® Flat Sheet Membranes (Celgard Inc., Charlotte, N.C., USA).
Components present in the feed, the diameter of which is too great to pass through the membrane pores are retained on the membrane surface and remain in part adhering. In crossflow filtration, feed permanently flows over the membrane surface in order to transport away the retained components (retentate) from the membrane surface. In this manner, continuous filtration operation with constant permeate flux is possible. The crossflow mode of operation results in the typical structure of membrane modules having three connections or passages, for feed, retentate and permeate. Membrane modules are equipped with a housing or frame which is closed or open on one side or many sides, and in which flat filter elements, or in rare cases wound filters, are mounted. According to the structure, a membrane module, in addition to passages between the filter elements or passages between the windings of the wound filter, has connections optionally arranged on the walls of the housing for feed, retentate and permeate.
In a flat filter element, the permeate space is bordered by two separate membranes or by two part-surfaces of a one-piece membrane. Between the two membranes or part surfaces, a porous permeate spacer is arranged which firstly acts as support structure for the sensitive membranes which are loaded by a transmembrane differential pressure of up to 100 bar, and secondly provides passageways through which the permeate runs off along the insides of the membranes/part-pieces. In a membrane module having a plurality of flat filter elements, the permeate space is composed of the totality of the permeate spaces of all flat filter elements.
In flat filter modules, a multiplicity of planar flat filter elements is arranged in a stack in parallel to one another. Between each two adjacent flat filter elements spacers are arranged which keep a passageway open through which the feed and retentate can flow in and off. The spacers comprise, e.g., washers made of a polymeric material which are arranged between the rim regions or edges, in particular the corners, of each two adjacent flat filter elements. Alternatively thereto, a frame or housing can be used which is equipped with equidistant grooves for receiving the edges of the flat filter elements.
An important area of use of filtration devices having flat filter modules is membrane bioreactors (MBR) for wastewater treatment. In MBR processes, the wastewater is treated in a plurality of steps physically, chemically and biologically until it reaches the membrane. By mechanical-physical pretreatments, the wastewater is freed from particles, fibers and coarse materials. In the coarse filtration, large particles, which can cause damage to the membranes, are removed by grills and sieves. In the MBR process, usually fine sieves are used for the prefiltration in a size range from 0.05-3 mm. The wastewater is in addition freed from heavy particles (e.g. sand) and oils and fats by a sand and fat trap.
In a further process step, the wastewater is treated biologically and chemically. In an activation tank, sludge (biomass) is situated together with microorganisms which enzymatically react and eliminate high-molecular-weight organic pollutants. The substances remaining after the enzymatic reaction are utilized by the microorganisms either for cell buildup or for energy production with oxygen consumption. The resultant oxygen consumption needs to be met by a sufficient oxygen supply, for which reason activation tanks are provided with aeration units. A precondition for the functioning of the process is the dwelling of the biomass in the system. Therefore, the biomass is separated off from the purified wastewater by a membrane filtration and recirculated to the activation tank. Overgrown activated sludge is removed as excess sludge. Before the biomass is separated from the water, further chemical treatments are performed. In this case, in combination with a filtration stage, usually various precipitants and flocculants such as, for example, iron chloride or polymers, are used for removing liquid components dissolved colloidally and as particles.
An essential advantage of MBR systems is the solids-free effluent. This means that no bacteria are situated in the effluent of the membrane activation system and even viruses may optionally be separated off by sorption effects. The residual organic fouling is strongly reduced thereby. The hygienically relevant guide values of the EU Bathing Water Directive [75/160/EEC, 1975] are complied with by MBR. In addition, the solids-free effluent offers great potential for wastewater reuse in the municipal and also industrial sectors. Here, by water recycling up to closing water circuits, large savings in water are achieved. A further advantage is that this process, owing to the adjustable high DM content and dispensing with secondary clarification tanks, has only a low space requirement. On account of the non-dependence on the sedimentation behavior, the activated sludge concentration (biomass concentration, expressed as DM—dry matter) can be increased compared with conventional processes. Membrane bioreactors are usually operated with DM concentrations from 8 to 15 g/l. Compared with conventional activation processes, the reactor volume of a membrane bioreactor can be reduced in such a manner that high space loadings are possible.
One problem with use of membrane filters in the field of wastewater purification is what is termed “membrane fouling” which is that deposits form on the membranes which reduce the flow of the liquid that is to be purified.
In the prior art, a multiplicity of filtration devices having flat filter modules and gas lifting systems are known.
EP 1 445 240 (whose United States equivalent is U.S. Pat. No. 6,245,239) describes a biological membrane reactor having a cyclically operated aeration system. The reactor comprises a feed-filled tank having one or more membrane modules which comprise optionally flat filter modules made of flat filter elements arranged vertically and at a distance from one another. Air is supplied to the feed using a cyclically operated aeration system. The aeration system comprises aeration nozzles which are arranged in the tank beneath the flat filter modules.
The filtration devices that are known in the prior art have some disadvantages:                in order to achieve good filtration efficiency, a feed volume is required that is at least twice as high as the free flow volume of the filter modules of the filtration device; correspondingly, the filtration devices have a space or area requirement which corresponds to about twice that of their base area;        for generating a sufficiently intensive crossflow streaming, a large liquid volume is circulated, in such a manner that in running filtration operations, considerable amounts of energy are consumed;        mechanical in-situ cleaning of the filtration membranes by granules is problematic, because a considerable part of the granules, owing to the crossflow streaming, sediments in the clarification tank/filtration tank surrounding the filtration device and/or flows off therefrom and downstream pumps are damaged in the long term.        